The exhaust gases emitted from internal combustion engines of vehicle, such as automobiles, have been purified by the exhaust-gas-purifying catalysts. Nowadays, in the exhaust-gas-purifying catalysts, ternary catalyst components have been mainly used to simultaneously remove hydrocarbons (HC), carbon monoxides (CO) and nitrogen oxides (NOx) included in the exhaust gases.
As for an exhaust-gas-purifying catalyst with a ternary catalyst component, it is common firstly to form a loading layer that is formed by fireproof inorganic oxides such as alumina, ceria-zirconia and the like on a surface of a catalyst support substrate, which is composed of a honeycomb substrate formed by cordierite or metals; and secondly to support platinum, palladium, rhodium and the like on the loading layer.
For example, one of the exhaust-gas-purifying catalysts described above, has been disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 10-296085. The exhaust-gas-purifying catalyst disclosed in the Japanese Unexamined Patent Publication (KOKAI) No. 10-296085, is characterized by a catalyst support substrate, on which two or more of the catalyst-coating layers are formed. The upper layer (surface) of the catalyst-coating layers includes: a fireproof inorganic oxide which is mainly composed of activated alumina; at least one additive member or more selected from the group consisting of cerium, zirconium or their compounds; and rhodium. The lower layer of the catalyst-coating layers includes: a fireproof inorganic oxide which is mainly composed of activated alumina; cerium; zirconium; palladium and barium.
However, the exhaust-gas-purifying catalyst mentioned above has a problem in that, the catalytic metal becomes inactivated because of the growth of the particle size or the occurring of the solid solution in the case that the catalytic metals are exposed to high temperature during the time of purifying the exhaust gases.